Nice job. Agree about stitching lines around inlays and increased spacing. Might also want to skive down the rectangles so there isn't quite the lip around the inlays. Can't wait to see your next project.
It appears there are multiple "styles" on the page. The one in question has holes for the strap to pass through. I placed matching arrow colors to show those that go together. Each different color is a different style, or pattern.
lambskin will work. You could also try some 1-2 oz. vegtan leather. Either one will prevent more thickness or stiffness. Make sure to sew this on as well, not just glue it, then make sure to burnish the edges for that finished look.
welcome to the forum. It is a folded edge to provide a little more rigidity to the edge of the wallet. When using chrome tan leathers, it is preferable to do this. Otherwise it is pretty soft and can become misshapen easily.
If the ebay stamps don't suit your liking, there is a place in Oklahoma who makes custom stamps (if they don't already have the design you are looking for).
https://www.shop.bunkhousetools.com/Embossing-Plates_c4.htm
Those look fantastic! Love the color. Stitching looks on point. When I back stitch at the end of a run, I will pull the first stitching really tight so the back stitch over the top will lay down a bit more and not be as obvious. Make sure to burnish those belt slots too.
Great idea to use the bison as the gussets. It probably made it easier to sew / bend around. Very sharp. Well done on the lettering too. Very difficult to do and do right.